The present invention relates to a coextrusion method for forming multiple layered plastic film and the products formed therefrom. In particular, this invention relates to a process, and the products formed thereof, for forming a lenticular-coated substrate, a lenticular strippable plastic-coated substrate or a nonlenticular strippable plastic-coated substrate that involves coextruding a molten thermoplastic tie resin and a molten thermoplastic lenticular resin onto a plastic substrate film. This invention further relates to a process, and the three-dimensional image formed thereof, for producing a three-dimensional image that entails printing an image onto the coextruded lenticular-coated substrate from gravure print cylinders that have been engraved with a gravure dot pattern having line spacings identical to the lenticular pattern of the coextruded lenticular-coated substrate, which lenticular pattern is cut into the chill roll with a precision gravure engraving machine, so that the gravure dot pattern is in exact registration with the lenticular pattern.
In general, known extrusion processes for making multilayered lenticular-coated substrates are limited to either multi-pass extrusion coating processes, or tandem extrusion coating processes. In either case, each particular layer, such as a tie layer or a lenticular layer, is extruded onto a substrate or a previously layered substrate followed by passage of the resulting product over a chill roll, such as a tie layer chill roll or a lenticular layer chill roll, to set the particular layer prior to extrusion of a subsequent layer. Thus, a separate chill roll is needed for each layer that is extruded onto the substrate and each layer is chilled appropriately prior to extrusion of a subsequent layer.
However, in either multi-pass or tandem extrusion coating processes, because each layer is cooled prior to the extrusion of the next layer, the processes inherently produce a product that contains bubbles or voids between the various layers. Consequently, when the multi-layered lenticular-coated substrates are employed to produce a three-dimensional image, these defects lead to a end product with lowered optical quality because the bubbles or voids distort the photographic imagery or lead to undesired star patterns in over-laminated or direct printed imagery.
Furthermore, the additional cost attributable to the presence of multiple heating and cooling cycles employed in multi-pass or tandem extrusion processes is undesired. Also, because these processes employ multiple heating and cooling cycles, flexibility with respect to the chemical composition and/or thickness of the various extruded layers is limited. For instance, a minimum thickness for the lenticular layer in either a tandem or multi-pass extrusion system is necessary to reduce the number of voids between the tie and lenticular layers. This lack of flexibility leads to less than optimum control over the handability and curl of the resulting product.
Lenticular-coated substrates can also be made on sheet lines or the lenticular layer can be cast using thermosetting resins. However, in the case of sheet line lenticular, the resin systems used usually include slip additives whose addition can lead to serious problems, such as lenticular-coated substrates in which surfaces are not optimally receptive to ink and/or adhesive promoters, when the product is used in various end applications. Furthermore, a sheet line process leads to unrestrained shrinkage during the cooling cycle and, thus, deformation of the lenses. Casting of thermoset resins is undesirable because this process is relatively slow and employs expensive resins.
One particular end application of lenticular-coated substrate involves the ability to strip the tie and lenticular layers, which are bonded together as a unit, from the substrate after the product is formed. These products are particularly useful to produce photographic masters. In this application, the master is photographically shot through the strippable lenticular. The lenticular layer is then peeled off and the "fuzzy" photographic reproduction is used to produce the four color separation for printing. A variant of this end application involves the production of a strippable multi-layered non-lenticular-coated substrate in which the lenticular layer (alternatively referred to as the carrier layer) does not contain a lenticular pattern. These products are particularly useful for producing merchandising material that offers a peelable advertising device, such as coupon material. These lenticular and non-lenticular strippable films are capable of being processed in other operations, such as slitting, printing or adhesive bonding, prior to being separated along the interface between the substrate and the tie layer.
Conventional techniques for manufacturing these strippable films rely on the use of lacquer-coated adhesive bonding or pattern adhesives using weak adhesives. However, these techniques result in products that possess nonuniform properties, such as nonuniform optical patterns or variable bonding of the tie/lenticular layers to the substrate. This nonuniformity produces distracting results to the viewer; patterned adhesives are not suitable for any optical application because of this nonuniformity.
Another end application of lenticular-coated substrate involves the use of the lenticular-coated substrate in three-dimensional imagery produced by gravure printing. In order to produce a three-dimensional image by this technique, registration of the print pattern to the lenticular pattern is necessary. Conventional methods of three-dimensional gravure printing include maintaining the registration of the gravure dot patterns and the lenticular pattern by mechanical means on the press without exact registration of all tooling from start to finish. Alternatively, the image can be printed and then over-laminated, again controlling the registration manually.
However, these methods produce three-dimensional images of less than optimal optical quality because of inexact registration. In particular, moire patterns are a typical defect in these products. Furthermore, because the gravure dot patterns cannot be accurately matched to the lenticular pattern, low lense numbers, typically 80 to 120 lpi, are used to allow for this error, leading to further degradation of the optical image. In addition, the low lense count employed necessitates the use of thicker lenticular layers, leading to cost inefficiencies.
Thus, there exists a need for a process for forming a lenticular-coated substrate which capitalizes on the advantages of an extrusion-based system but yet produces a product that can be manufactured to a higher quality at a lower cost, particularly for use of in three-dimensional photographic, computer or printed image processes. In particular, there exists a need for an extrusion process which minimizes the presence of bubbles or voids between the layers while increasing flexibility with respect to the chemical composition and thickness of the layers. Ideally, the shape, thickness and lenticular line count can be varied extensively to suit the ultimate end application of the lenticular-coated substrate formed without degrading the optical qualities of the lenticular-coated substrate.
Additionally, there exists a need for a process for forming a strippable plastic film, either a lenticular-coated substrate or a nonlenticular-coated substrate, which produces a product with uniform lenticular patterns or bonding characteristics, particularly for use in optical applications.
Further, there exists a need for a process of producing a three-dimensional gravure-printed image in which exact registration of the gravure dot pattern and the lenticular pattern is maintained to allow for manufacture of an image of high optical quality. In particular, there exists a need for a process capable of producing low-cost three-dimensional images that are substantially devoid of moire patterns and which exhibit accurate and reproducible colors. Additionally, there exists a need for a process capable of producing three-dimensional images printed at high resolution, particularly higher than 500 dots per inch.